Suggested doc improvement: Add note that KUBE_* vars only available in jobs with an environment

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https://docs.gitlab.com/ee/user/project/clusters/#deployment-variables

Would be helpful to indicate here that the KUBE_* deployment variables described in this section are only available if the env is set. Perhaps something like:

NOTE: The following deployment variables are only available to jobs that have environments set. You can read more about environments and find examples in the documentation about environments.